Ink jet printing is a non-impact method for recording information in response to an electronic signal, such as that generated by a computer. In the printer, the electronic signal produces droplets of ink that are deposited on a substrate such as paper or transparency film. Ink jet printers have found broad commercial acceptance due to their reliability, relatively quiet operation, print quality, and low cost. Thermal or bubble jet printers have been especially successful and have found broad application as output devices for personal computers in the office and the home.
Both dyes and pigments have been used as colorants for ink jet inks. While the dye-based inks in general are superior in color properties, they have several disadvantages as compared to pigment-based inks. The dyes are usually water soluble and remain so after drying on the substrate. They are easily redissolved by water spills, and the image smears on contact with the felt pen markers. In addition, the dyes exhibit poor light stability relative to pigments and are known to fade even under office lighting. Thus, dye-based inks are often unsuitable for use in applications requiring moisture resistance and greater light stability. The pigments are preferred colorants provided the pigment dispersion can be made resistant to flocculation and settling.
Aqueous pigment dispersions are well known and have been used commercially in coating applications such as paints. A pigment dispersion is generally stabilized by either a non-ionic or ionic technique. In the non-ionic technique, the pigment particles are stabilized by a dispersant compound that has a water-soluble, hydrophilic section that extends into the aqueous medium and provides entropic or steric stabilization. Representative compounds useful for this purpose include polymers of polyvinyl alcohol, cellulosics, and ethylene oxide modified phenols. While the non-ionic technique is not sensitive to pH changes or ionic contamination, it has a major disadvantage for many applications in that the final product is water sensitive.
In the ionic technique, the pigment particles are stabilized by a dispersant compound containing ionizable groups, such as acrylic, maleic, or sulfonic acid. The compound provides stabilization through a charged double layer mechanism whereby ionic repulsion hinders the particles from flocculation. The non-ionic form of the dispersant, which remains after the ink is dry, have reduced water solubility, resulting in an ink which is not moisture sensitive. In addition, the dispersant should also have strong binding to the pigment surface.
Polymer dispersants having both random and block structures are known in the art as effective for use in ink jet inks.. For example, U.S. Pat. No. 4,597,794 teaches an aqueous ink for ink jet printers in which the pigment is contained in a polymer having anionic hydrophilic segments and aromatic hydrophobic segments that adhere to the pigment surface. U.S. Pat. No. 5,085,698 teaches the use of AB or BAB block polymers as pigment dispersants for aqueous ink jet inks. While it is easier and cheaper to make the random polymers, they are not as effective as the block polymers in stabilizing the pigment dispersion. The block polymers, however, usually require special synthetic techniques such as anionic or group transfer polymerization.